1. Field of the Invention
The present invention relates to a method of fabricating a semiconductor device. More particularly, the present invention relates to a method of removing spacers and fabricating metal-oxide-semiconductor (MOS) transistors.
2. Description of the Related Art
Metal-oxide-semiconductor (MOS) transistor is an important device in a semiconductor. FIG. 1A is schematic cross-sectional view of a conventional MOS transistor. As shown in FIG. 1A, a MOS transistor 120 is usually disposed on a substrate 100 (for example, a polysilicon substrate). The MOS transistor comprises a gate structure 122, a source region 124s, a drain region 124d and spacers 126. The gate structure 122 is disposed over the substrate 100. The gate structure 122 comprises a gate dielectric layer 122a and a gate layer 122b sequentially stacked over the substrate 100. The source region 124s and the drain region 124d are disposed in the substrate 100 on the two sides of the gate structure 122, respectively. The spacers 126 are disposed on the sidewalls of the gate structure 122.
To improve the mobility of electrons and holes in the channel region (the region underneath the gate structure 122), the spacers 126 are often removed using phosphoric acid in a wet etching operation after the process for fabricating the MOS transistor 120 is completed. Thereafter, a strain layer 130 is formed over the MOS transistor 120 and the substrate 100 to adjust the lattice structure of the substrate 100 (as shown in FIG. 1B).
However, electrons and holes would traverse the PN junction between the source region 124s and the substrate 100 as well as the drain region 124d and the substrate 100. When phosphoric acid is used to remove the spacers 126, the phosphoric acid and the electrons and holes at the PN junction would trigger a photo-electrochemical reaction in the presence of light. Ultimately, the source region 124s and the drain region 124d are damaged. This damaging effect is more serious for an NMOS transistor. FIG. 2 is a photo of an NMOS transistor taken by a scanning electron microscope (SEM) showing the damages in the source and the drain region when the spacers are removed using phosphoric acid in the presence of light. As shown in FIG. 2, voids 128 (indicated by arrows) are formed in the source region 124s and the drain region 124d when the spacers 126 are removed using phosphoric acid in the present of light.